(1) Field of the Invention
This invention relates to electromagnetic energy absorbers and, more particularly, to electromagnetic energy absorbers molded of conductive loaded resin-based materials comprising micron conductive powders, micron conductive fibers, or a combination thereof, homogenized within a base resin when molded. This manufacturing process yields a conductive part or material usable within the EMF or electronic spectrum(s).
(2) Description of the Prior Art
Electromagnetic energy is an important consideration in the art of electrical and electronic devices, and particularly in the art of communications devices. A normally operating electronic device will typically emit significant amounts of stray electromagnetic energy due electrical charge accelerations occurring in the device. This energy radiates out from the device at the speed of light. The vast majority of this energy is simply dissipated in the surrounding atmosphere. However, if another electronic device is operating near the radiating device, then a part of the radiated energy may well intersect with this second device. This electromagnetic energy, in the form of electric and magnetic fields, can interact with the second electronic device to create interference. That is, the electromagnetic energy becomes an unwanted, interference signal in the second device. This interaction is called electromagnetic interference (EMI) or radio frequency interference (RFI).
From the standpoint of electrical and electronic design, the electromagnetic energy interactions in the above-described scenario create two broad considerations. First, proposed designs must take into account the electromagnetic energy environment in which the proposed devices will operate. Much as the design must take into account the operating temperature range, so the design must also take into account the range of frequencies, magnitudes, and durations of electromagnetic sources in the operating environment. While the above discussion describes the case of electromagnetic radiation emanating from a nearby device, other potential sources must also be accounted for. For example, electrical power grids, passing motor vehicles, cellular mobile phones, personal computers, and other man-made devices all emit electromagnetic energy during normal operation. Further, natural sources, such as the sun, generate electromagnetic energy that can create signal interference in the proposed device. Any of these sources in the operating environment can generate electromagnetic energy that can be coupled into the proposed device through cables, circuit board traces, or routing, connectors, packaging, materials, and the like.
Second, the design must take into account the electromagnetic energy that can be emitted from the proposed device through the operating circuits, devices, routings, connectors, cables, and the like. It is not sufficient to simply ‘bullet proof’ the proposed design from outside interference. The design must also meet specifications limiting the content of the electromagnetic energy emitted from the device. These specifications are developed to insure that all systems in the overall operating environment function properly. Further, these specifications may be promulgated by government or regulatory agencies and carry the force of legal requirements. For example, the U.S. Federal Communications Commission establishes guidelines limiting the content of electromagnetic energy emissions for various types of manufactured devices sold in the U.S. Market in order to protect publicly accessed communications frequencies.
The above described considerations are most pronounced in devices that use electromagnetic energy to perform a critical function. For example, a radar tracking system installed on a naval ship transmits pulses of intense electromagnetic energy and then monitors, or receives, components of this energy that are reflected back to the ship by solid objects, such as other ships. This type of radar tracking system may be adversely affected by interactions between the transmitted radar energy and structures on the ship such as masts, towers, artillery tubes, and the like. As a result, significant degradation in tracking ability may occur. In this scenario, and in many wireless communication scenarios, the design must trade off the need for large and well-defined electromagnetic energy levels required for transmitting data or for detection waves with the need to limit the power of transmitted energy to avoid EMI/RFI issues or to avoid health concerns. Further, the design must trade off the need to detect weak electromagnetic signals with the need to block other signals.
The above-described considerations have resulted in the creation in the art of numerous structures, techniques, and methods to handle EMI/RFI issues experienced in the field. Further, with the advent of careful government regulation and of customer specification, the analysis and testing of electromagnetic capabilities in manufactured devices is now an important consideration. Testing, such as measuring the effect of a well-defined interference signal or pattern impinging on a device, requires equipment and facilities to carefully generate a specified set of conditions, such as signal strength and frequency, and to measure the response of a device under test (DUT) to those conditions. Anechoic chambers have been constructed wherein the interior spaces are carefully shielded from external sources of electromagnetic energy. Further, electromagnetic energy radiating from sources inside the anechoic chambers, such as radiating test antennas or even the DUT, itself, is quickly absorbed. A number of structures, devices, and methods have been developed for testing electromagnetic energy issues.
Several prior art inventions relate to devices or materials to control electromagnetic energy. U.S. Patent Publication U.S. 2003/0117787 A1 to Nakauchi teaches a shield for reducing radio frequency (RF) interference associated with an electronic circuit. U.S. Patent Publication U.S. 2004/0020674 A1 to McFadden et al teaches a composite EMI shield comprising a conductive layer and an absorptive layer. The conductive layer in this invention is selected from the group consisting of silver, nickel, copper, aluminum, steel, silver/glass, graphite, carbon, conductive polymers, and combinations thereof. The absorptive layer is then applied by spraying or dipping an electromagnetic energy absorptive particle onto the conductive layer. U.S. Pat. No. 6,479,140 B1 to Takao et al teaches radio wave absorbing materials comprising conductive filler, an inorganic endothermic filler, and an organic binder as main constituents. The preferred conductive filler in this invention is carbon black. U.S. Pat. No. 6,007,905 to Kudo et al teaches a wave absorber comprising a layer of a foamed thermoplastic organic polymer, a conductive layer, and a third layer of the foamed thermoplastic organic polymer. The conductive layer that is taught in this invention is a mixture of a conductive carbon black, conductive graphite, and a latex of a thermoplastic organic polymer. U.S. Patent Publication U.S. 2003/0198800 A1 to Hoffman teaches a sheet like plastic element for the confinement of high frequency reflections. Electrically conductive particles are mixed into polyurethane backing material.
U.S. Patent Publication U.S. 2003/0146866 A1 to Hayashi et al teaches a radio wave absorber unit for use in an anechoic chamber. This invention teaches a polypropylene-based conductive expanded bead for the substrate material with the beads ranging in size from 2 mm to 10 mm. U.S. Patent Publication U.S. 2003/0108744 A1 to Kuchler et al teaches the use of highly porous glass granulate and/or ceramic granulate coated with ferrite and/or an electrically conductive material as a filler in the production of an electromagnetic absorber material. This filler is mixed with mortar and used for forming walls or absorber linings or housings. U.S. Patent Publication U.S. 2003/0119459 A1 to Carillo et al teaches the use of an electromagnetic field shield that is reflective, absorptive or dissipative in nature, to block a direct line-of-sight electromagnetic field radiating from an antenna of a wireless device. U.S. Pat. No. 6,359,581 B2 to Kurihara et al teaches an electromagnetic wave absorber for use in an anechoic chamber. The absorber includes a wave absorbing section comprising an array of plate-shaped elements of magnetic loss material where the spaces between each plate-shaped element comprise a dielectric loss material. The magnetic loss material may comprise a resin containing ferrite powder. The dielectric loss material may comprise a foam, or a resin, containing carbon or graphite.